1. Field of the Invention
The present invention relates to a linear image sensor IC made up of a plurality of light receiving elements that output an output signal in response to a quantity of received light, a plurality of switch elements which are connected between output terminals of adjacent light receiving elements, a sample/hold circuit that holds an output of the light receiving elements, and a scanning circuit that reads electric charges held by the sample/hold circuit under control, and more particularly to a contact type image sensor, an image scanner, a facsimile machine or a copying machine which can change the resolution.
2. Description of the Related Art
For example, JP 5-227362 A has proposed a contact type image sensor in which a control terminal is additionally disposed for resolution control so that a user can change over the resolution in accordance with the applied conditions.
FIG. 11 is a circuit diagram showing a contact type image sensor integrated circuit that has been proposed in the above-mentioned publication. In this conventional art, a control terminal 125 is disposed in an image sensor chip, and the user inputs a signal of a high level or a low level to the control terminal, to thereby realize a changeover of the resolution between a high resolution mode and a low resolution mode. The outline of FIG. 11 will be described now. A start pulse SI and a clock pulse CLK are inputted to a shift register group 104. When a shift register 104a starts due to the start pulse SI, an output of the shift register 104a is supplied to a channel select switch 103a through a NOR gate 121a and an AND gate 120a to turn on the channel select switch 103a, and the cannel select switch 103a picks up a signal from a photo cell 101a to a signal line 107a. Other shift registers 104b to 104f also sequentially start and output signals from the respective photo cells 101b to 101l to the signal lines 107a and 107b. In this example, when a control signal H is inputted to the control signal input terminal 125, analog switches 110a, 110b, 122a and 122b change over so that an image signal with a read density of 16 dots/mm can be obtained at an image output terminal 111. Also, when a control signal L is inputted to the control signal input terminal 125, the analog switch 110a is constantly in an ON state, to thereby obtain an image signal with the read density of 8 dots/mm at an image output terminal 111 which is the half of the entire photo cells 101a to 101l. That is, although all of the photo cells 101a to 101l on a sensor IC are constantly operating, output image signals can be partially thinned out and outputted in accordance with the control signal when the output image signals are picked up to the external. For that reason, a voltage level of the image signal is always kept constant, and an image processing circuit at a post stage can be structured by the conventional circuit.
Since the conventional image sensor IC is structured as described above, the data of the skipped output signal of the light receiving element may be missing because the data is thinned out when the resolution is made rough.